function setRem() {
    //ui尺寸
    let uiSize = 750
    //理想视口尺寸
    let winSize = document.documentElement.clientWidth
    //375 iphone6的宽度
    let app = document.getElementById('app')
    let footer = document.querySelectorAll('.page-footer')[0]
    //如果视口尺寸大于设计尺寸750时，就显示750宽度范围
    if (winSize > uiSize) {
        app.style.width = uiSize + 'px'
        app.style.margin = '0 auto'
        app.style.border = '1px solid black'
        return
    }
    //给html元素设置font-size值
    document.documentElement.style.fontSize = winSize / uiSize * 100 + 'px'
    app.style.width = winSize + 'px'
    app.style.border = 'none'
    // footer.style.width = winSize + 'px'
}

//当页面加载的时候执行的函数
onload = function () {
    setRem()
}
//当页面缩放的时候
onresize = function () {
    setRem()
}